1. Field of the Invention
The present invention relates to a control device for an ultrasonic motor which is suitable for use in controlling the speed of the ultrasonic motor.
2. DESCRIPTION OF THE RELATED ART
There has hitherto been a control device wherein when an ultrasonic motor is moved to a target position, the speed at which the ultrasonic motor travels is gradually slowed as it approaches the target position, whereby the ultrasonic motor is stopped accurately at the target position (refer to Japanese Unexamined Patent Application No. 1-325257). The travel speed of the ultrasonic motor is slowed in the following way. The position of the ultrasonic motor is detected by a position sensor or similar equipment. The duty ratio of a control pulse is gradually altered from 100% to 0% as the detected position approaches the target position; this control pulse being ANDed with a drive signal at a resonant frequency of the ultrasonic motor. The time during which the drive signal is fed is then interrupted incessantly.
In such a control device for an ultrasonic motor, the duty ratio of the control pulse is uniformly set with respect to the detected position of the ultrasonic motor. Therefore, when the speed of the motor varies due to factors such as fluctuations in load, it is difficult to correct the travel speed of the ultrasonic motor to an optimum value.